1. Field of the Invention
The present invention relates to electronics and electrical systems. More specifically, the present invention relates to delta-sigma modulators.
2. Description of the Related Art
Analog to digital converters are widely used for converting analog signals to corresponding digital signals for many electronic circuits. For example, a high resolution analog to digital converter (ADC) may find application in radar, missile, and communications systems. There are two basic techniques for performing analog to digital conversion: an open-loop technique and a feedback technique. An open-loop ADC generates a digital signal directly in response to an analog input signal. This approach uses precisely matched components to digitize the input signal. The resolution and accuracy of the open-loop ADC depend on the matching of these components. However, highly precise components are difficult to achieve in conventional integrated circuit processing.
A delta-sigma (ΔΣ) ADC (also known as a sigma-delta ADC) is a feedback type of ADC that subtracts a feedback signal from the analog input signal to provide an error signal, which is filtered and then quantized to form a digital output signal. The delta-sigma approach achieves high resolution by precise timing instead of by precisely matched components (such as resistors and capacitors) that are required in open-loop converters. The delta-sigma technique is therefore the preferred technique for many applications.
A delta-sigma ADC typically includes a delta-sigma modulator and a decimator. The delta-sigma modulator (also known as a sigma-delta modulator) uses oversampling (having a sampling rate above the Nyquist rate) and filtering to develop a high signal-to-noise ratio in the signal band. The decimator then resamples the output of the modulator and provides an N-bit data word at the Nyquist rate.
A simple delta-sigma modulator includes a quantizer a filter; and a digital to analog converter (DAC). The quantizer generates a digital output signal in response to filtered difference between the analog input signal and a feedback signal. The feedback signal is the digital output signal reconverted to analog by the DAC. The filter shapes the quantization noise to frequencies outside of the signal band. Thus, the delta-sigma converter is referred to as a noise-shaping ADC. The decimator typically includes a filter having a lowpass (or bandpass) characteristic with a cutoff frequency at (or within a bandwidth of) the Nyquist frequency. Since the sampling frequency is much higher than the Nyquist frequency, the filter can usually attenuate this out-of-band quantization noise sufficiently.
Delta-sigma ADCs were originally developed for digitizing baseband (low pass) signals, such as audio signals. Since delta-sigma ADCs typically operate at clock frequencies below 100 MHz, and a large oversampling ratio (i.e., the sampling rate to the Nyquist rate) is required to obtain high resolution, sigma-delta ADCs have traditionally been employed to digitize analog signals below 1 MHz. Bandpass delta-sigma modulators are a relatively new idea intended to apply delta-sigma techniques to intermediate frequency (IF) signals. A bandpass delta-sigma modulator is designed to suppress quantization noise over a narrow band of frequencies centered at an intermediate frequency (from about 1 MHz to 2 GHz).
Delta-sigma modulators are difficult to implement at high frequencies using active components. Passive inductive-capacitive (LC) resonators have therefore been used to implement bandpass delta-sigma modulators. Conventional LC bandpass delta-sigma modulators, however, require multiple feedback paths having different delays. This approach causes large voltage swings in the loop filters and leads to higher distortion and less dynamic range. In addition, all the analog components and the feedback DACs need to have very low distortion and low noise, resulting in increased circuit complexity.
Hence, a need exists in the art for an improved bandpass delta-sigma modulator that offers higher resolution, better linearity, and less circuit complexity than conventional bandpass delta-sigma modulators.